Re: [RFC PATCH] staging, android, ashmem: invalidate pin/unpin ioctl for private map

[Dan Carpenter <dan.carpenter@oracle.com>]

On Mon, Dec 03, 2012 at 09:09:59AM +0900, JoonSoo Kim wrote:
> Hello, Dan.
> 
> 2012/12/2 Dan Carpenter <dan.carpenter@oracle.com>:
> > On Sat, Dec 01, 2012 at 02:45:57AM +0900, Joonsoo Kim wrote:
> >> @@ -614,21 +616,35 @@ static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
> >>       pgstart = pin.offset / PAGE_SIZE;
> >>       pgend = pgstart + (pin.len / PAGE_SIZE) - 1;
> >>
> >> -     mutex_lock(&ashmem_mutex);
> >> +     if (asma->shared_mapping) {
> >> +             mutex_lock(&ashmem_mutex);
> >
> > Wouldn't we need to hold the mutex while we check the
> > ->shared_mapping?
> 
> I doesn't fully understand ashmem's lock semantic.
> Code for retrieving some value of asma instance doesn't hold the mutex, now.
> For example, in ashmem_ioctl(), asma->size, asma->prot_mask.
> And in ashmem_pin_unpin(), there is asma->file, asma->size which is
> retrieved without the mutex.
> According to this semantic, the mutex doesn't need for checking
> asma->shared_mapping.

The ashmem_ioctl() is clearly racy.  :P  asma->size can be modified
and read at the same time.  It's not an example to follow.

regards,
dan carpenter